Seismic surveying uses an artificially induced seismic wave to infer underground geological formations. A seismic wave source approximates a delta function by striking the surface or setting off an explosion. Receivers detect the local pattern of ground motion over a short interval of time following the triggering of the seismic source. An amplified output of each receiver is recorded as a seismic trace, which represents the combined response of the layered subsurface and the recording system to the seismic source.
The response of solid rock formations in the earth to various types of applied seismic body waves is well known. A recent development in seismic surveying has been the use of three-component geophones to collect information from each type of body wave. These three-component geophones obtain data that permits particle motion to be represented as a three-component vector, with the motion being in response to compressional waves, vertical shear waves, and horizontal shear waves.
Regardless of the type of waves being detected, as in any transmission system, the signals generated by the seismic source undergo filtering by the earth during transmission to the receiver. The effect of such filtering is considered to be "noise", adverse to the desired seismic data. Thus, interpretation of seismic traces requires that filtering effects be reconciled.
One significant filtering effect that greatly affects the integrity of seismic recordings is the effect of the near surface layer. This near surface layer has properties that are very different from those of the consolidated rocks farther below. For example, the soil and the near surface rocks are affected by the elements, i.e., rain, frost, ice, temperature, and wind, which have long term as well as short term effects. Because of these different properties, the effect of the near surface layer on seismic waves is different from that of the underlying rock, and, unlike the response of seismic waves in solid rock, the response in the near surface is not well known.
Another characteristic of the near surface is that its effect is greater on shear waves than on compressional waves. Furthermore, this near surface effect is relatively overwhelming compared to the relatively subtle effects of the rock formations, with the latter being of interest to seismologists. The result is that the increasing use of multicomponent receivers has led to efforts to eliminate or reconcile the effect of the near surface. One method teaches burying the receivers below the near surface layer to avoid the near surface filtering. Another method teaches using both buried sensors and surface receivers. The buried sensor is under the source and measures the near surface effect so that a deconvolution operator is derived, which is then used to eliminate the near surface effect from the signal received at the surface. A problem with both methods, however, is that exploration results have shown that they do not consistently account for the actual behavior of seismic waves in the near surface. Thus, a need exists for an improved means for determining the effect of the near surface layer on seismic waves.